The invention pertains to high velocity air web dryers, for example, which force heated air transversely across the moving web to be dried, and air bars having nozzles for directing the heated air are positioned transversely along each of opposite sides of the moving web. Thus the web is suspended without contact while it moves through the dryer. During web offset printing, the web is printed on both sides at the same time, and then enters the flotation-type dryer at about 80.degree. F., exits about 260.degree. to 325.degree. F. and then wraps around a number of large diameter water cooled chill rolls where the thermoplastic ink is "set" by cooling the web to about 90.degree. F. or less. In typical web offset printing applications currently in use, higher press speeds, increased ink lay-down, and longer dryers have led to greatly increased lengths of unsupported webs. The unsupported web length is defined as the length of the web between the last printing unit which the web passes through, and the tangent point where the web first makes contact with a chill roll.
When presses are printing two webs at one time, the top web must be printed, then raised above the remaining printing units by using air support turning devices such as shown in the Peekna, U.S. Pat. No. 4,182,472 (FIG. 1), entitled "Contactless Turning Guide for Running Webs". For a typical 40 foot long flotation dryer, the lower web may have an unsupported length of about 50 feet whereas the upper web may have an unsupported length of approximately 75 feet. The longer the length of unsupported web, the more susceptible it is to minor forces that can cause web weave or web shift; that is to say, cause the web to move laterally from one side to the other from its normal longitudinal centerline of travel, or the web may shift to one side or the other and remain there.
Offset press speeds of 2000-2500 feet per minute are common. These high speeds require dryers 30 to 40 feet in length or more, depending upon the basis weight of the paper being printed and dried. Long lengths of unsupported web are adversely affected by laterally moving air currents, unevenly applied ink and water which causes the web to have baggy edges, or uneven or insufficient web tension throughout the press. When any or all of the above circumstances occur, the web will weave back and forth in a side to side motion after it exits from the dyer and attempts to wrap the first chill roll.
Web weave or shift generally occurs during start-up when the press first goes on impression. The web quickly absorbs water and expands in length, decreasing tension between the last printing unit and the chill roll stand. The web sometimes wanders from side to side on the chill rolls, anywhere from +/-1/16" to +/-1" and this is commonly referred to as web "weave". Or it may move suddenly to one side or the other, up to 3" or more and stay there, and this is defined as web shift. Unless the web can be brought back to longitudinal center position, the press must be shut down and the start-up procedure repeated. If the web weave is in the top web, many printers take manual control of a web guiding device in an attempt to keep web weave or shift to a minimum until the press can get up to speed.
Web weave causes waste at the folder end of the press because the printed signatures do not fold over in the same place every time and must be discarded. It is estimated that web weave contributes an additional 1% to 21/2% to the direct paper waste factor.
Assuming that each web break caused by web weave results in 30 minutes of press downtime, and that such web breaks average 11/2 per shift, we have a total of 375 hours per year in press downtime. At a rate of $1000/hour, the dollars lost per year would amount to $375,000 for press downtime. Add to this a 11/2% web waste factor caused by web weave and the total loss to a printer can easily amount to $582,000 per year for a 38" wide two-web press using 50 pound coated stock.
In many cases the web weave or shift is so fast that automatic web steering devices do not react quickly enough to keep the web on track. Such rapid web movement commonly occurs when the press printing units are first put on impression, and ink and water is applied to the web. This moistening of the web causes the paper fibers to expand and can result in a large loss of web tension and/or a "baggy edge".
Web weave and/or shift can occur when going on impression, during the blanket wash cycle, during increases in press speed, during a flood cycle or during a splice. It can and does exist anywhere between the last print unit and the cutoff cylinder located in the folder.
On some occasions, the web will move to one side or the other and stay virtually locked at its new position. The new position may not be "centered" on the press and will prevent the press from producing an acceptable product. To force the web into its centered position, it is sometimes necessary to apply adhesive type tape to the first chill roll to increase its diameter, increase web tension and help force the web to move in the desired direction to a new permanent position.
If the web weave or shift is severe enough, the web may move off the edge of the chill roll and cause the web to break or tear. This in turn forces a press stoppage in order to rethread the paper web through the press. Each web break of this nature will cause a loss of production time of approximately 15-45 minutes. It is often necessary to make two or three attempts to get the press up to speed before the operators are successful in steadying the web movement to the point where the web can successfully be fed all the way through the press.
Any lateral air movements within the dryer will cause the web to move in the direction of air flow and this effect is greater with lower web tension or with a baggy edge. Cross machine air flow patterns are very undesirable.
Some prior art flotation dryers (see FIG. 3) have generally installed air flow devices so as to force the flow of spent air (return air) along a path starting at the centerline of the web being dried and then traveling in a path perpendicular to said centerline to spent air (exhaust air) slots extending the full length of the dryer at the front and back edges of the web. The slots are of a width as determined by calculation and extend parallel to the web edges, front and backside, and sometimes in the center of the dryer.
Other prior art dryers use an open return air concept wherein there is essentially no blockage of the return or spent air after it has impinged on the web, such as shown in British Pat. No. 1,067,918 of May 10, 1967, where the spent air passes transversely and off the web edges. The British Pat. No. 634,244 to Spooner, published Mar. 15, 1950, is not a web floater device, but is a web against a cylinder arrangement. Furthermore, it simply removes the spent air from the same side of the web at which the support jets are located, so that the spent air does not interfere with the support jets of air. These prior art dryers do not control return air flow so as to prevent web weave by avoiding lateral forces (transverse to the direction of web travel) on the web.
Although some prior art was capable of successful production when the dryer and press are completely threaded and up to speed, the problems caused during initial start-up and when making paper roll splices continue to cause frequent web breaks and press downtime. Operating costs remain high on other dryers due to web breaks and downtime caused by web weave or shift.
The prior art does not successfully overcome the problems of web weave or web shift, but many attempt to eliminate web flutter, stretch the web being treated or avoid nozzle interference. See, for example, U.S. Pat. No. 3,680,223 to Vits in which an elaborate return air system is suggested.
With the use of a contactless turning guide for running webs such as in U.S. Pat. Nos. 4,197,972 issued Apr. 15, 1980 to Daane, or 4,182,472, issued Jan. 8, 1980 to Peekna, in web offset presses the problem of web weave became prevalent and web waste increased.
A relationship exists between the length of unsupported web and its width and its susceptibility to web weave or web shift. In the printing industry, where web tensions are relatively high (about 2.0 pounds per lineal inch and above), unsupported web lengths greater than 5.times.W' where W' equals the web width in feet, will usually have web weave and web shift problems.
Still other examples of the prior art are shown in U.S. Pat. No. 3,739,491 which issued June 19, 1973 to Creapo et al and is entitled "High Velocity Air Web Dryer". This patent discloses the general arrangement of air bars contemplated by the present invention, the use of headers for supplying the air to said air bars, the insulated oven which contains the air bars and headers, and suitable oven doors, blowers, and exhaust ducts. U.S. Pat. No. 4,197,973, issued Apr. 15, 1980 to Daane, relates to high velocity air bars having air flow straightening means for the discharge slots which attempt to dissipate cross machine momentum components of air movement.